.\" Copyright (c) 2006,2008-2011 Joseph Koshy.  All rights reserved.
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.\" $Id: elf_begin.3 3639 2018-10-14 14:07:02Z jkoshy $
.\"
.Dd December 11, 2011
.Dt ELF_BEGIN 3
.Os
.Sh NAME
.Nm elf_begin
.Nd open an ELF file or ar(1) archive
.Sh LIBRARY
.Lb libelf
.Sh SYNOPSIS
.In libelf.h
.Ft "Elf *"
.Fn elf_begin "int fd" "Elf_Cmd cmd" "Elf *elf"
.Sh DESCRIPTION
Function
.Fn elf_begin
is used to open ELF files and
.Xr ar 1
archives for further processing by other APIs in the
.Xr elf 3
library.
It is also used to access individual ELF members of an
.Xr ar 1
archive in combination with the
.Xr elf_next 3
and
.Xr elf_rand 3
APIs.
.Pp
Argument
.Ar fd
is an open file descriptor returned from an
.Xr open 2
system call.
Function
.Fn elf_begin
uses argument
.Ar fd
for reading or writing depending on the value of argument
.Ar cmd .
Argument
.Ar elf
is primarily used for iterating through archives.
.Pp
The argument
.Ar cmd
can have the following values:
.Bl -tag -width "ELF_C_WRITE"
.It ELF_C_NULL
Causes
.Fn elf_begin
to return NULL.
Arguments
.Ar fd
and
.Ar elf
are ignored, and no additional error is signalled.
.It ELF_C_READ
This value is to be when the application wishes to examine (but not
modify) the contents of the file specified by the arguments
.Ar fd
and
.Ar elf .
It can be used for both
.Xr ar 1
archives and for ELF objects.
.Pp
If argument
.Ar elf
is NULL, the library will allocate a new ELF descriptor for the file
being processed.
The argument
.Ar fd
should have been opened for reading.
.Pp
If argument
.Ar elf
is not NULL, and references a regular ELF file previously opened with
.Fn elf_begin ,
then the activation count for the descriptor referenced by argument
.Ar elf
is incremented.
The value in argument
.Ar fd
should match that used to open the descriptor argument
.Ar elf .
.Pp
If argument
.Ar elf
is not NULL, and references a descriptor for an
.Xr ar 1
archive opened earlier with
.Fn elf_begin ,
a descriptor for an element in the archive is returned as
described in the section
.Sx "Processing ar(1) archives"
below.
The value for argument
.Ar fd
should match that used to open the archive earlier.
.Pp
If argument
.Ar elf
is not NULL, and references an
.Xr ar 1
archive opened earlier with
.Fn elf_memory ,
then the value of the argument
.Ar fd
is ignored.
.It Dv ELF_C_RDWR
This command is used to prepare an ELF file for reading and writing.
This command is not supported for
.Xr ar 1
archives.
.Pp
Argument
.Ar fd
should have been opened for reading and writing.
If argument
.Ar elf
is NULL, the library will allocate a new ELF descriptor for
the file being processed.
If the argument
.Ar elf
is non-null, it should point to a descriptor previously
allocated with
.Fn elf_begin
with the same values for arguments
.Ar fd
and
.Ar cmd ;
in this case the library will increment the activation count for descriptor
.Ar elf
and return the same descriptor.
.Pp
Changes to the in-memory image of the ELF file may be written back to
disk using the
.Xr elf_update 3
function.
.It Dv ELF_C_WRITE
This command is used when the application wishes to create a new ELF
file.
Argument
.Ar fd
should have been opened for writing.
Argument
.Ar elf
is ignored, and the previous contents of file referenced by argument
.Ar fd
are overwritten.
.El
.Ss Processing ar(1) archives
An
.Xr ar 1
archive may be opened in read mode (with argument
.Ar cmd
set to
.Dv ELF_C_READ )
using
.Fn elf_begin
or
.Fn elf_memory .
The returned ELF descriptor can be passed into to
subsequent calls to
.Fn elf_begin
to access individual members of the archive.
.Pp
Random access within an opened archive is possible using
the
.Xr elf_next 3
and
.Xr elf_rand 3
functions.
.Pp
The symbol table of the archive may be retrieved
using
.Xr elf_getarsym 3 .
.Sh RETURN VALUES
The function returns a pointer to a ELF descriptor if successful, or NULL
if an error occurred.
.Sh EXAMPLES
To iterate through the members of an
.Xr ar 1
archive, use:
.Bd -literal -offset indent
Elf_Cmd c;
Elf *ar_e, *elf_e;
\&...
c = ELF_C_READ;
if ((ar_e = elf_begin(fd, c, (Elf *) 0)) == 0) {
	\&... handle error in opening the archive ...
}
while ((elf_e = elf_begin(fd, c, ar_e)) != 0) {
	\&... process member referenced by elf_e here ...
	c = elf_next(elf_e);
	elf_end(elf_e);
}
.Ed
.Pp
To create a new ELF file, use:
.Bd -literal -offset indent
int fd;
Elf *e;
\&...
if ((fd = open("filename", O_RDWR|O_TRUNC|O_CREAT, 0666)) < 0) {
	\&... handle the error from open(2) ...
}
if ((e = elf_begin(fd, ELF_C_WRITE, (Elf *) 0)) == 0) {
	\&... handle the error from elf_begin() ...
}
\&... create the ELF image using other elf(3) APIs ...
elf_update(e, ELF_C_WRITE);
elf_end(e);
.Ed
.Sh ERRORS
Function
.Fn elf_begin
can fail with the following errors:
.Bl -tag -width "[ELF_E_RESOURCE]"
.It Bq Er ELF_E_ARCHIVE
The archive denoted by argument
.Ar elf
could not be parsed.
.It Bq Er ELF_E_ARGUMENT
The value in argument
.Ar cmd
was unrecognized.
.It Bq Er ELF_E_ARGUMENT
A non-null value for argument
.Ar elf
was specified when
.Ar cmd
was set to
.Dv ELF_C_RDWR .
.It Bq Er ELF_E_ARGUMENT
The value of argument
.Ar fd
differs from the one the ELF descriptor
.Ar elf
was created with.
.It Bq Er ELF_E_ARGUMENT
Argument
.Ar cmd
differs from the value specified when ELF descriptor
.Ar elf
was created.
.It Bq Er ELF_E_ARGUMENT
An
.Xr ar 1
archive was opened with
.Ar cmd
set to
.Dv ELF_C_RDWR .
.It Bq Er ELF_E_ARGUMENT
The file referenced by argument
.Ar fd
was empty.
.It Bq Er ELF_E_ARGUMENT
The underlying file for argument
.Ar fd
was of an unsupported type.
.It Bq Er ELF_E_IO
The file descriptor in argument
.Ar fd
was invalid.
.It Bq Er ELF_E_IO
The file descriptor in argument
.Ar fd
could not be read or written to.
.It Bq Er ELF_E_RESOURCE
An out of memory condition was encountered.
.It Bq Er ELF_E_SEQUENCE
Function
.Fn elf_begin
was called before a working version was established with
.Xr elf_version 3 .
.It Bq Er ELF_E_VERSION
The ELF object referenced by argument
.Ar fd
was of an unsupported ELF version.
.El
.Sh SEE ALSO
.Xr elf 3 ,
.Xr elf_end 3 ,
.Xr elf_errno 3 ,
.Xr elf_memory 3 ,
.Xr elf_next 3 ,
.Xr elf_rand 3 ,
.Xr elf_update 3 ,
.Xr gelf 3
